restricted access III.17 Alternative Stable States and Regime Shifts in Ecosystems
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III.17 Alternative Stable States and Regime Shifts in Ecosystems Marten Scheffer OUTLINE 1. Introduction 2. The theory in a nutshell 3. Shallow lakes as an example 4. Mechanisms for alternative stable states in ecosystems 5. How to know if a system has alternative stable states 6. Using alternative stable states in management Complex systems ranging from cells to ecosystems and the climate can have tipping points, where the slightest disturbance can cause the system to enter a phase of selfpropagating change until it comes to rest in a contrasting alternative stable state. The theory explaining such catastrophic change at critical thresholds is well established. In particular, an early influential book by the French mathematician René Thom catalyzed the interest in what he called ‘‘catastrophe theory.’’ Although many claims about the applicability to particular situations were not substantiated later, catastrophe theory created an intense search for real-life examples, much like chaos theory later. In the 1970s, C. S. Holling was among the first to argue that the theory could explain important aspects of the dynamics of ecosystems, and an influential review by Sir Robert May in Nature promoted further interest among ecologists. Nonetheless, not until recently have strong cases for this phenomenon in ecosystems been built. GLOSSARY alternative stable states. A system is said to have alternative stable states if under the same external conditions (e.g., nutrient loading, harvest pressure, or temperature) it can settle to different stable states. Although genuine ‘‘stable states’’ occur only in models, the term is also used more liberally to refer to alternative dynamic regimes. attractor. A state or dynamic regime to which a model asymptotically converges, given sufficient simulation time. Examples are a stable point, a cycle, or a strange attractor. catastrophic shift. A shift to an alternative attractor that can be invoked by an infinitesimal small change at a critical point known as catastrophic bifurcation . hysteresis. The phenomenon that the forward shift and the backward shift between alternative attractors happen at different values of an external control variable. regime shift. A relatively fast transition from one persistent dynamic regime to another. Regime shifts do not necessarily represent shifts between alternative attractors. resilience. The capacity of a system to recover to essentially the same state after a disturbance. 1. INTRODUCTION The idea of catastrophic change at critical thresholds is intuitively straightforward in physical examples. Suppose you are in a canoe and gradually lean over to one side more and more. It is difficult to see the tipping point coming, but eventually leaning over too much will cause you to suddenly capsize and end up in an alternative stable state from which it is not easy to return. Still, people have been hesitant to believe that large complex systems such as ecosystems or the climate would sometimes behave in a similar way. Indeed, fluctuations around gradual trends rather than ‘‘tipping over’’ seem the rule in nature. Nonetheless, occasionally sudden changes from one contrasting fluctuating regime to another one are observed. Such abrupt changes have been termed regime shifts. As we shall see in this chapter, regime shifts are indicative of the existence of tipping points and alternative stable states but by no means a proof. Indeed, rigorous experimental proofs are possible only in small controlled systems. Such a difficulty of proving that a mechanism is at work in nature is common in ecology. For instance, it has proven remarkably hard to demonstrate unequivocally the role of a mechanism as basic as competition. Nonetheless, the role of alternative stable states in driving some ecosystems dynamics has become an important focus of research. I first briefly show the key aspects of the theory and elaborate the case of shallow lakes as an example. Subsequently, I briefly highlight some other mechanisms and discuss how one may find out if a system has alternative stable states. Finally, I reflect on how insights in such stability properties can be used in managing ecosystems. 2. THE THEORY IN A NUTSHELL Smooth, Threshold, and Catastrophic Response to Change In most cases, the equilibrium of a dynamic system moves smoothly in response to changes in the environment (figure 1A). Also, the system is often rather insensitive over certain ranges of the external conditions , although it responds relatively strongly around some threshold conditions (figure 1B). For instance, mortality of a species usually drops sharply around a critical concentration of a toxicant. In such a situation, a strong response happens when a threshold is...